Download or read book Atomic-scale Investigation of Deformation Mechanisms in Concentrated Alloys written by Mulaine Shih and published by . This book was released on 2021 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Mechanical behavior of crystalline materials is determined by crystal defects such as dislocations. Understanding the response of defects to forces is thus crucial to improve mechanical properties and to better design and engineer advanced structural materials. Motivated by metastable high entropy alloys (HEAs), we first study the dislocation behavior in a class of metastable fcc alloys. Transitioning from fcc to hcp stable phases in these alloys corresponds to a change from positive to negative average stacking fault energy (SFE). We perform Molecular Dynamics (MD) simulations, and use random NiCo alloys as the model metastable fcc alloy system. We show that the splitting distance between Shockley partial dislocations can remain finite even in a negative average SFE alloy, at finite temperatures. By investigating the effect of average versus “local” SFE on the dislocation stability, and examining the decorrelation force to break the two partial dislocations from equilibrium in these alloys, we are able to piece the puzzle together. We show that in concentrated alloys, the major resisting force is caused by the interaction of dislocations with local solute environments acting on partial dislocations. Next, we present our effort to predict the stress state of atomic-scale defects in experimental observations, captured by high resolution transmission electron microscopy (TEM) images. We propose a new method utilizing the continuum J-integral concept to compute the forces at atomic-scale. The new method is applied to example atomistic simulations of dislocation interactions, and results are validated with its theoretical Peach-Koehler force. Moreover, we investigate the potential origin of preferential deformation at thin deformation twins in CrCoNi alloys under cyclic loading. Assisted from atomistic simulations, we examine the local stress field at the twin boundary with free surfaces. Pure Cu was chosen as our model system for its chemical simplicity and having a similar anisotropy ratio to CrCoNi alloys. We demonstrate the effect of twin size on the strain localization. The high elastic anisotropy induces local stress concentration, which served as a crack initiation site under cyclic loading. We then extend the crack initiation model proposed by Neumann and Tonnessen to discuss the behavior in CrCoNi alloys. In summary, this thesis interrogates the possible deformation mechanisms in concentrated alloys and provides fundamental understanding of the dislocation behavior and offers insight for future alloy developments and applications.